Relation stanza declarations and header tag

Made changes to conform to obo-format 1.4 and http://code.google.com/p/developmental-stage-ontologies/

 * Added an ontology tag. This should be 'wbls' (all lower case)
 * Added a stanza for part_of (see issue #1)
 * Added xref for PURL for preceded_by (but see also issue #2)
 * Removed stanza for is_a (this is a built-in predicate in obo-format)